Complexes of thioethers

ABSTRACT

Technetium-99m complexes of crown thioethers having 3 to 6 donor sulphur atoms joined into a macrocyclic ring by C2 or C3 groups, for example 1,4,7-trithiacyclononane (9S3); and of tripodal ligands having more than 3 donor sulphur atoms joined by C2 or C3 groups, for example, 1,1,1-tris(((2-methylthio)ethylthio)-methyl)ethane.

This is a continuation of PCT application No. PCT/GB91/00482, filed Mar.28, 1991.

This invention concerns technetium complexes of thioethers. In oneembodiment, the thioether is a crown thioether having three to six donorsulphur atoms joined into a macrocyclic ring by C2 or C3 groups.

In another embodiment, there is used a tripodal ligand having more thanthree donor sulphur atoms joined by C2 or C3 groups.

Technetium-99m is a γ-emitter having a half life of 6 hours, which isvery widely used for body imaging. Complexes according to the inventionin which technetium is Tc^(99m) are expected to have interestingbiodistribution properties and to constitute useful body imaging agents.

Tripodal ligands are typically 1,1,1-substituted ethanes, in which eachof the three substituents contains at least one sulphur atom. The threesubstituent groups may be different but are generally the same, andcontain between them more than three sulphur atoms.

Examples of crown thioethers and tripodal ligands are shown in FIG. 1.

In the crown thioether designations, the first figure represents thenumber of atoms, and the last figure the number of sulphur atoms, in themacrocyclic ring. Thus:

9S3=1,4,7-trithiacyclononane;

12S3=1,5,9-trithiacyclocododecane;

14S4=1,4,8,11-tetrathiacyclotetradecane;

15S5=1,4,7,10,13-pentathiacyclopentadecane;

18S6=1,4,7,10,13,16-hexathiacyclooctadecane;

L=1,1,1-tris(((2-methylthio)ethyl)thio)methyl)ethane.

The complex may comprise, associated with each Tc atom: one or twothioether ligands; 0, 1, 2, or 3 oxo groups; and 0, 1, 2 or 3 halide orpseudohalide groups (e.g. F, Cl, Br, I or NCS). The following diagramshows the Tc thioether complexes that are chemically plausible.Parentheses enclose the charges of species less likely to exist. Xdenotes halide or pseudohalide.

    __________________________________________________________________________    Oxidation State:                                                                            Tc(I)                                                                              Tc(II)                                                                            Tc(III)                                                                           Tc(IV)                                                                            Tc(V)                                                                             Tc(VI)                                                                            Tc(VII)                                __________________________________________________________________________    TcS.sub.6                                                                          [Tc(9S3).sub.2 ].sup.n+                                                                n = 1+                                                                             2+  3+                                                          [Tc(12S3).sub.2 ].sup.n+                                                               1+   2+  3+                                                          [Tc(18S6)].sup.n+                                                                      1+   2+  3+                                                          [Tc(L)].sup.n+                                                                         1+   2+  3+                                                          [Tc(14S4)].sup.n+                                                                           (2+)                                                                              (3+)                                                   TcOX.sub.2 S.sub.3                                                                 [TcOX.sub.2 (9S3)].sup.n+                                                                       (1-)                                                                              0   1+  2+  3+                                          [TcOX.sub.2 (12S3)].sup.n+                                                                      (1-)                                                                              0   1+  2+  3+                                     TcOS.sub.n                                                                         [TcO(14S4)].sup.n+                                                                              1+  2+  3+  4+  5+                                          [TcO(15S5)].sup.n+                                                                              (1+)                                                                              2+  3+  4+  5+                                     TcO.sub.2 XS.sub.3                                                                 [TcO.sub.2 X(9S3)].sup.n+                                                                       (-2)                                                                              1-  0   1+  2+                                          [TcO.sub.2 X(12S3)].sup.n+                                                                      (-2)                                                                              1-  0   1+  2+                                          TcO.sub.2 (14S4)].sup.n+                                                                    (-2)                                                                              1-  0   1+  2+                                         TcO.sub.3 S.sub.3                                                                  [TcO.sub.3 (9S3)].sup.n+  (1-)                                                                              (0) 1+                                          [TcO.sub.3 (12S3)].sup.n+ (1-)                                                                              (0) 1+                                     __________________________________________________________________________

Synthesis of Ligands

Ligand L was synthesized for the first time in the inorganic ChemistryLaboratory at Oxford University in 1986. This and subsequent workrevealed the efficacy of L as a ligand for Co(II) and Ni(II). Subsequentwork has shown that it also strongly binds Ru(II) to produce thehexacoordinated [Ru(L)]²⁺ cation.³

Crown thioethers ligands were prepared by routes that have now appearedin the literature. In general these involve reaction of thioetherdithiols with oligo(thioether) dichlorides in dimethylformamidecontaining Cs₂ CO₃ as base. Syntheses of 9S3 (Equation 1), 18S6(Equations 2a-2c), and supertripodal ligand L (Equations 3a-b) are shownbelow.

    HS(CH.sub.2).sub.2 S(CH.sub.2).sub.2 SH+Cl(CH.sub.2).sub.2 Cl+Cs.sub.2 CO.sub.3 →9S3                                      (1)

    HS(CH.sub.2).sub.2 S(CH.sub.2).sub.2 SH+2NaOEt+2Cl(CH.sub.2).sub.2 OH→HOCH.sub.2 (CH.sub.2 SCH.sub.2).sub.3 CH.sub.2 OH(2a)

    HOCH.sub.2 (CH.sub.2 SCH.sub.2).sub.3 CH.sub.2 OH+2SOCl.sub.2 →ClCH.sub.2 (CH.sub.2 SCH.sub.2).sub.3 CH.sub.2 Cl (2b)

    Cl(CH.sub.2).sub.2 S(CH.sub.2).sub.2 Cl+HS(CH.sub.2).sub.2 S(CH.sub.2).sub.2 SH+Cs.sub.2 CO.sub.3 →18S6       (2c)

    CH.sub.3 C(CH.sub.2 OH).sub.3 +3TsCl→CH.sub.3 C(CH.sub.2 OTs).sub.3 (3a)

    CH.sub.3 C(CH.sub.2 OTs).sub.3 +3CH.sub.3 S(CH.sub.2).sub.2 SH+3NaOEt→L                                        (3b)

Crown thioethers were first prepared over fifty years ago, but theircoordination chemistry was not examined until the early 1960s, and mostof their coordination chemistry has been investigated in the last tenyears. For example, the ligand 9S3 was prepared in 1977 and itscoordination dates back only to 1982. In that time crown thioethercomplexes of a number of transition metals, including particularly Ru(with 14S4 and 9S3) have been prepared.

In the light of this earlier work, it was unclear what oxidation stateof Tc would result in any complexes that might be formed when the ligandwas reacted with pertechnetate, and the biodistribution properties ofany resulting complexes were also unclear.

By analogy with earlier work, the crown thioethers should form Tcchelates that differ in redox potential (for the hexakis(thioether)series [Tc(L)₂ ]²⁺, L=9S3, 12S3, 18S6) and in coordination number (for[Tc(14S4)X_(m) ]^(n+), where m can be 1 or 2, and [Tc(15S5)X]^(n+)).Variation in redox potential provides an important means of tuning thecharge on the cation, and thereby influencing its biodistribution. Forexample, a chelate for which Tc(III/II) potential falls within thephysiological range would not localise cleanly in one tissue. Suitablechoice of a different ligand could be used to shift the potential out ofthe physiological range and thereby obviate this problem.

The availability of coordination sites on the Tc ion also influencesbiodistriution. Easily displaced X ligands (as opposed to the crownthioether, which is more difficult to displace) allows the biologicalmilieu to interact with Tc chelates (e.g., those used for bone imaging).This interaction (with proteins thiolate groups, for example) canstrongly influence biodistribution. The crown thioethers have beenchosen either to saturate the Tc coordination sphere (9S3, 12S3, 18S6)or to not to do so (14S4, 15S5) in order to examine both types ofbehaviour.

Synthesis and Characterisation of the Tc Chelates

Reaction of n-Bu₄ NTcO₄ (85 mg, 0.21 mmol) with the crown thioether 9S3(160 mg, 0.9 mmol) in MeCN (5 ml) and HBF₄ (0.4 mL of a 54% solution inEt₂ O) added to give a dark brown solution. After stirring for 1 h abrown microcrystalline precipitate formed, which was washed with Et₂ Oto give the product (115 mg, 79%). Elemental analysis indicates theformula [Tc(9S3)₂ ](BF₄).2H₂ O.1/2MeCN, a previously unreportedcompound. (Anal. Calc. (found) for C₁₃ H₂₉.5 N₀.5 O₂ S₆ B₂ F₈ Tc: C22.76 (22.63), H 5.16 (4.34), N 0.99 (1.01).

Addition of saturated aqueous solution of NH₄ PF₆ to the BF₄ ⁻ saltdissolved in 5 mL H₂ O precipitated a golden brown solid that wasfiltered, washed with H₂ O and Et₂ O and dried in air. Elementalanalysis indicates the formula [Tc(9S3)₂ ](PF₆).H₂ O, also a previouslyunreported compound. (Anal. calc. (found) for C₁₂ H₂₆ OS₆ P₂ F₁₂ Tc: C18.75 (18.78); H 3.12 (3.41).

Crystals formed upon slow diffusion of Et₂ O into solutions of the BF₄ ⁻or PF₆ ⁻ salts in CH₃ CN or CH₃ NO₂. During the crystallisationsometimes became pale red, which may correspond to a product resultingfrom either solvolysis (particularly hydrolysis by adventitious water)or slow oxidation by air.

Electrochemistry

Electrochemical characterisation of [Tc(9S3)₂ ]²⁺ by cyclic voltammetry(MeCN, Pt electrode, 0.1 M Bu₄ NPF₆ electrolyte) shows aquasi-reversible wave (probably corresponding to the Tc (II/I) couple)at +0.05V vs the saturated calomel electrode (SCE; -0.38V vsferrocenium/ferrocene (Fc⁺ /Fc), +0.02 vs standard hydrogen electrode,(SHE). It also shows an irreversible one-electron oxidation at +1.3 V vsSCE (+0.87 V vs Fc⁺ /Fc, +1.27 V vs SHE).

Another redox couple yields an irreversible reduction at -1.6 V vs SCE(-2.03 V vs Fc⁺.Fc, -1.63 V vs. SHE). Owing to its much smaller peakcurrent than that of the waves described above, this redox processprobably results from either an impurity or from a reaction product of[Tc(9S3)₂ ]²⁺ under the experimental conditions.

In addition, a quasi-reversible process appears at -0.66V vs SCE (-1.09V vs Fc⁺ /Fc, -0.69 V vs SHE). On the first scan (100 mV/s) this wavehas a current one-fourth that of the wave at +0.05V vs SCE. Duringsubsequent scans the current associated with this wave at -0.66 V vs SCEgrew at the expense of that at +0.05 V vs SCE; by the tenth scan thesystem reaches either an equilibrium or a steady state. These resultssuggest that upon reduction of [Tc(9S3)₂ ]²⁺ to [Tc(9S3)₂ ]⁺ (theprocess at +0.05 V) the electrogenerated [Tc(9S3)₂ ]⁺ reacts (perhaps)with CH₃ CN to give a product that is redox-active in its own right (andwhich gives rise to the wave at -0.66 V).

Cyclic voltammetry in aqueous solution employed 0.1 M NaCl as supportingelectrolyte; other conditions were as described above. Twoquasi-reversible redox processes occur, one at -0.20 V vs SCE (+0.02 Vvs. NHE) and another at -0.60 V vs. SCE (-0.37 V vs. NHE). The peakcurrent of the second wave depends on the number of scans. On successivescans it grew to an equilibrium or steady state value about half that ofthe wave at -0.20 V.

Supertripodal Ligand L

Parallel synthetic procedures with the supertripodal compound L yieldedevidence for the analogous Tc(II) complex. Under the conditionsdescribed above L reacts with TcO₄ ⁻ to yield a solution identical inappearance with that of the 9S3 homologue. Consequently it is formulatedas [Tc(L)]²⁺, which also has not been previously reported.

Analogy with the known coordination chemistry of these ligands suggestsfor them the structures shown in FIG. 2, namely [Tc(9S3)₂ ²⁺ and [TcL]²⁺respectively.

Biodistribution Data for [^(99m) Tc(9S3)₂ ]^(n+) (n=1, 2)

[^(99m) Tc(9S3)₂ ]⁺ was made by SnCl₂ reduction of [^(99m) Tc(9S3)₂ ]²⁺.Biodistribution was determined in female CD mice at 1 h post injectiongiven as mean %ID/g (S.D.) (N=2 except as indicated).

    ______________________________________                                                 [.sup.99m Tc(9S3).sub.2 ].sup.2+                                                           [.sup.99m Tc(9S3).sub.2 ].sup.n+                        ______________________________________                                        blood      1.53     (0.23)    1.33   (0.43)                                   urine                                                                         feces      1.38     (0.54)                                                    heart      0.6      (0.12)    0.76   (0.09)                                   lungs      2.17     (0.24)    2.46   (0.19)                                   thyroid    1.11     (N = 1)   1.43   (N = 1)                                  liver/gall bladder                                                                       14.45    (1.68)    12.28  (1.59)                                   stomach    0.52     (0.07)    0.80   (0.21)                                   spleen     1.59     (1.11)    1.51   (0.01)                                   gut/pancreas                                                                             6.11     (0.31)    10.28  (0.60)                                   kidney     17.2     (0.28)    15.78  (1.92)                                   bladder    2.4      (N = 1)   1.40   (0.04)                                   brain      0.13     (0.09)    0.06   (0.01)                                   muscle (thigh)                                                                           0.35     (0.14)    0.25   (0.06)                                   bone (femur)                                                                             0.56     (0.18)    0.32   (0.06)                                   adrenal    0.58     (N = 1)   1.46   (0.21)                                   % retained >57,     >53       >60,   >76                                      ______________________________________                                    

We claim:
 1. A Technetium complex of a crown thioether having 3 to 6donor sulphur atoms joined into a macrocyclic ring by ethylene ortrimethylene groups.
 2. A complex as claimed in claim 1 wherein thecrown thioether is 1,4,7-trithiacyclononane.
 3. A complex as claimed inclaim 2 having the formula [TcL₂ ]⁺ or [TcL₂ ]²⁺ or [TcL₂ ]³⁺ where L isthe crown thioether.
 4. A Technetium complex of a 1,1,1-substitutedethane in which each of the three substituents contains at least onesulphur atom having more than three donor sulphur atoms joined byethylene or trimethylene groups.
 5. A complex as claimed in claim 4,wherein the ligand is 1,1,1-tris(((2-methylthio)ethylthio)-methyl)ethane.
 6. A complex as claimed in claim 5 having the formula [TcL]⁺ or[TcL]²⁺ or [TcL]³⁺ where L is the ligand.
 7. A complex as claimed in anyone of claims 1 to 6, wherein the Technetium is Technetium-99m.